The present invention relates generally to devices for transmitting electromagnetic signals of a desired frequency between a source and a load and more particularly to devices for transmitting electromagnetic signals of a desired frequency between a source and a load that additionally provide over-voltage protection to the transmission line.
A radio frequency (RF) transmission line is a structure that is designed to efficiently transmit high frequency radio frequency (RF) signals between a source and a load. An RF transmission line typically comprises two conductors, such as a pair of metal wires, that are separated by an insulating material with dielectric properties, such as a polymer or air. One type of an RF transmission line which is well known in the art is a coaxial electric device.
Coaxial electric devices, such as coaxial cables, coaxial connectors and coaxial switches, are well known in the art and are widely used to transmit electromagnetic signals over 10 MHz with minimum loss and little or no distortion. As a result, coaxial electric devices are commonly used to transmit and receive signals used in broadcast, military, police, fire, security and civilian transceiver applications as well as numerous other uses.
A coaxial electric device typically comprises an inner signal conductor which serves to transmit the desired communication signal. The inner signal conductor is separated from an outer conductor by an insulating material, or dielectric material, the outer conductor serving as the return path, or ground, for the communication signal. Such an electric device is typically referred to as coaxial because the inner and outer conductors share a common longitudinal axis. It should be noted that the relationship of the geometry of the conductors and the properties of the dielectric materials disposed between the conductors substantially defines the characteristic impedance of the coaxial device.
It has been found that, on occasion, potentially harmful voltages are transmitted through RF transmission lines. In particular, radios operating in either the lower end of the ultra high frequency (UHF) band or lower frequency bands (i.e., below 500 MHz) often utilize longer antenna lengths to enhance performance compared to antennae used in higher frequency applications. In addition, the long range signal propagation characteristics of these lower frequencies allow for superior long range communication. Furthermore, since the mounting height of a radio antenna serves to increase its range, radio antennae are commonly mounted from an elevated position (e.g., a tower or mast). As a result, it has been found that radio antennae are highly susceptible to lightening strikes, the high electrical energy of a lightning strike increasing the likelihood of significant damage to any sensitive components connected to the transmission line, which is highly undesirable.
As a result, at least one RF transmission line component is commonly provided with protective means for deflecting undesirable electromagnetic impulses away from a load connected thereto. As will be described in detail below, a number of different means for protecting an RF transmission line from over-voltage is well-known in the art.
As a first means for protecting an RF transmission line from over-voltage, at least one transmission line component is provided with a device that conducts if the voltage transmitted therethrough exceeds a pre-determined threshold (e.g., a metal oxide varistor (MOV) or similar solid state device), the device in turn being connected directly to ground. Although useful in deflecting undesirable impulses away from a load connected to the transmission line, these types of protective devices carry a relatively high capacitance which in turn limits its operation to relatively low frequencies (i.e., frequencies under 1 MHz).
As a second means for protecting an RF transmission line from over-voltage, at least one transmission line component is provided with a shunt conductor which connects the center conductor to either the outer conductor or ground. The operational frequency of protective devices which utilize shunt conductors is typically greater than 400 MHz because lower frequencies require excessively long shunt conductors. As can be appreciated, the use of excessively long shunt conductors is disfavored, among other reasons, for substantially increasing the overall size of the protective device. An example of a protective device provided with a shunt conductor for grounding undesirable impulses is shown in U.S. Patent Application Publication No. 2004/0169986 to George M. Kauffman, which is hereby incorporated by reference.
As a third means for protecting an RF transmission line from over-voltage, at least one transmission line component is provided with a single gas discharge tube (GDT) that avalanches or conducts transient, high voltage impulses from the center conductor to ground. It should be noted that gas discharge tubes are characterized as having (i) a relatively high transient current capacity, (ii) a compact design and (iii) an inexpensive construction, all of which are highly desirable. For at least these reasons, it has been found that the gas discharge tube is the preferred means in the art for protecting RF transmission lines from over-voltage in components designed to operate at frequencies below 400 MHz.
Although well known in the art, transmission line components which utilize a single gas discharge tube often suffer from a notable drawback. Specifically, it has been found that components which utilize a single gas discharge tube offer a limited lifespan of full functionality. For example, a single heavy duty gas discharge tube can only survive a single impulse of 30 kA. Once the gas discharge tube fails, the protective component requires expensive replacement and/or repair. Otherwise, devices and circuits connected to the transmission line are rendered susceptible to damage from future impulses.